Blender Soft Body deforming too much
Soft Body
To keep shape uncollapsed can be achieved by tweaking some parameters, but it will be always more &quot;softer&quot; in a center and &quot;harder&quot; in corners where springs are closer to each other without additional geometry (see Nathan's answer).
There is also a big game changer - topology. In your specific case if you go to edit mode select all and search for Un-Subdivide like 2, than under Physics Properties &gt; Soft Body &gt; Field Weights &gt; Gravity set back to 1 ... you can see object doesn't collapse.

Cloth
You can try Cloth simulation instead, that has an Internal Springs feature. Here I scaled bag to something more real-size (1 x 0.5 x 0.3). Un-Subdivided to 3. Physics Properties &gt; Vertex Mass 10 kg, enabled Internal Springs. Add Collision modifier with Thickness Outer 0.001 and Friction 50.